The present invention relates to a process for preparing isocyanates.
To prepare isocyanates by phosgenating the corresponding amines, there is in principle the possibility of a liquid phase phosgenation or of a gas phase phosgenation. In the gas phase phosgenation, the reaction conditions are selected such that at least the diamine, diisocyanate and phosgene reaction components, but preferably all reactants, products and reaction intermediates, are gaseous under these conditions, more preferably until the reaction is complete. The present invention relates exclusively to gas phase phosgenation. In this context, “essentially” means to an extent of at least 50% by weight based on all reaction components, preferably to an extent of at least 66%, more preferably to an extent of at least 75%, even more preferably to an extent of at least 85%, in particular to an extent of at least 90% and especially to an extent of at least 95%. The temporary formation of liquid droplets and/or temporarily formed particulate solids which react in a gaseous environment would be conceivable.
EP 1 275 639 A1 describes the gas phase phosgenation of (cyclo)aliphatic diamines in a reaction zone with constrictions of the walls.
In the mixing device, the amine- and phosgene-containing reactant streams are fed coaxially to a mixing zone, the phosgene-containing reactant stream being conducted in the interior and the amine-containing reactant stream in the exterior. In the region in which the reactant streams are combined, i.e. the reaction zone, there is a further reduction or slight enlargement of the flow cross section, such that the flow rate rises owing to the volume increase in the course of the reaction as a result of expansion of the gas.
A disadvantage of this arrangement is that the amine stream is conducted coaxially in the exterior. This can result in solid formation on the walls of the mixing device, since the amine is present in excess compared to the phosgene at the walls, which promotes by-product formation.
It is likewise stated in EP 1275639A1 that swirling of the reactant streams should be effected in the mixing apparatus before the reactant streams are combined such that the turbulent variable speeds in the reactant flows are then increased and the mixing is then effected more rapidly when the two reactant streams are combined.
EP 1526129 A1 describes the increase in turbulence in a mixing nozzle by swirl-generating internals. This generates tangential vortexing of the overall stream, which does not, however, have a significant effect on the mixing of the different streams with one another.
EP 1 275 640 A1 describes the gas phase phosgenation of (cyclo)aliphatic di- and triamines in a mixing tube with a reactor, in which the gas flow in the mixing region is accelerated.
A disadvantage of this process is that the maximum speed difference between the reactant streams is not achieved immediately at the start of mixing, and hence the minimum possible mixing time is not achieved either.
DE 10359627 A1 discloses a gas phase phosgenation in which amine is mixed in by means of a concentric ring gap between two phosgene streams, where the areas through which the phosgene streams flow are in a ratio of from 1:0.5 to 1:4.
International application WO 2007/028715 discloses a process in which amine and phosgene are metered in through an annular gap, i.e. a ring-shaped continuous gap.
In all of these documents, exclusively smooth nozzles are disclosed; turbulence-generating internals are disclosed at best by means of twisted arrangements.
None of the mixing apparatus known in the prior art cited has to date had the effect of permanently and satisfactorily suppressing the formation of solids at the point of combination of the amine stream and of the phosgene stream.
It was thus an object of the present invention to develop a reaction regime for a gas phase phosgenation, with which industrial scale performance is possible and which becomes insusceptible to blockages by virtue of there being only a small tendency to deposit solids.